On which listener event do i trigger alternate row coloring to tree grid

Hi,

I have a tree panel, with a tree store. Basically a tree grid same as the example of ext js 4. I have a written a code to have all rows of tree grid in alternate colors at any state. I mean on collapse of some node or expand of some node or expand or collapse of tree on whole. But im not sure on which event this should be triggered. Please look at the attached code. The listener should get called every time i expand or collapse some node. But only once after i do expand or collapse of tree using collpaseAll() or expandAll().